1. Field of the Invention
The invention is related to the field of communications, and in particular, to systems and methods of compensating for polarization-mode dispersion (PMD) mid-link on an optical communication network. More particularly, the invention is related to systems and methods of compensating for polarization-mode dispersion (PMD) and providing channel equalization simultaneously mid-link on an optical communication network.
2. Description of the Prior Art
Polarization-mode dispersion (PMD) is a dynamic pulse broadening phenomena. In a single-mode optical fiber, optical pulses propagating down the fiber will separate into two orthogonal modes of polarization that travel at different speeds. The relative amplitudes of these two pulses are determined by the state of polarization of the input pulse relative to the fiber's input principal states of polarization (PSP). The separation into the two orthogonal modes is caused by the non-uniformity of the core diameter of the fiber or birefringence of the fiber.
If the core has a perfectly circular cross-section, then both modes travel at the same speed over the same distance. Otherwise, one mode travels slower than the other mode resulting in a difference in group velocities (an effect called birefringence). Like chromatic dispersion, the difference in group velocities between polarization modes is wavelength dependent. For PMD, the difference in group velocity is also time dependent. The difference in propagation time, Δτ, experienced by the two polarization modes at a given wavelength is referred to as the differential group delay (DGD) with units in picoseconds (ps). When the DGD in a fiber becomes excessively large, the receiver is unable to distinguish between a zero bit and a one bit, and bit errors eventually occur resulting in a PMD-induced outage.
To compensate for PMD, optical PMD compensators are installed in the optical communication network at the receiver end of the network. In a Wavelength Division Multiplexed (WDM) network, PMD compensators are installed at the receiver end between the de-multiplexer (that de-multiplexes the individual channels) and the receiver. There is usually one PMD compensator for each channel. The PMD compensators mitigate the effects of PMD on the individual channels so that the receiver can more accurately decode the optical signals.
Network operators may compensate for PMD mid-link using optical-to-electrical-to-optical (O-E-O) regenerators. However, O-E-O regenerators are expensive and it may not be economically feasible to install O-E-O regenerators wherever there is a need for PMD compensation.
Unfortunately, optical PMD compensators are not currently installed mid-link in a network in an effective manner. One reason why PMD compensators are not currently installed mid-link is that PMD compensators are wavelength dependent and the WDM signals traveling over the network need to be separated or de-multiplexed into individual channels for the PMD compensators. Thus, network operators either need to install a new de-multiplexer to separate the wavelengths for the PMD compensators, or install the PMD compensators at mid-link points in the network where de-multiplexers are already installed.
Installing a new de-multiplexer/multiplexer combination mid-link for the purpose of installing PMD compensators may not be desirable. Installing a new de-multiplexer/multiplexer combination adds cost to the network. Also, the new de-multiplexer and the new multiplexer may attenuate the optical signals more than desired.
Similarly, installing current PMD compensators at mid-link points in the network where de-multiplexers are already installed may not be desirable. For instance, periodically over long haul and ultra-long haul networks, the gains of the individual wavelengths are equalized with a Dynamic Gain Equalizer (DGE). A DGE comprises a de-multiplexer, channel equalization elements, and a multiplexer. When the DGEs are installed in the network at gain equalization points, the de-multiplexer is installed ahead of the channel equalization elements to separate the WDM signals into individual channels and the multiplexer is installed following the channel equalization elements to combine the channels again. PMD compensators are generally not used in combination with a DGE to compensate for PMD mid-link because of the attenuation added by the PMD compensators. As an example, the combination of the de-multiplexer (˜5 dB loss), the channel equalization element (˜5 dB loss), the PMD compensator (˜10 dB loss), and the multiplexer (˜5 dB loss) could reach approximately 25 dB of attenuation which may be undesirable.
A problem still exists to effectively compensate for PMD mid-link in an optical communication network.